Many attempts have been made for converting methane to higher hydrocarbons, particularly to ethylene. As a matter of fact, the direct conversion of methane to ethylene is highly desirable, since ethylene may be used as raw material for the synthesis of various important products.
Much research has have been devoted to the oxidizing methane coupling reaction according to the scheme: ##STR1##
This reaction may be performed, depending on the type of catalyst, either in the simultaneous mode (passing a CH.sub.4 +O.sub.2 mixture over a catalyst) or the sequential mode (methane oxidation to higher hydrocarbons by means of a reducible agent, followed by the reoxidation of said agent by oxygen of air).
For the oxidizing coupling in the sequential mode, two patents (U.S. Pat. No. 4,499,323 and U.S. Pat. No. 4,523,049) indicate the use of praseodymium oxide, Pr.sub.6 O.sub.11, as reducible agent at a temperature from 700.degree. to 800.degree. C.
For the oxidizing coupling in the simultaneous mode, certain works indicate the use of rare-earth oxides, particularly: Otsuka and Coll. in a series of articles (Chemistry Letters, 1985, p. 499; 1986, p. 467; 1987, p. 483; J. Catalysis, 1986, 100, p. 353) and Imai in an article (J. Chem. Soc. Chem. Comm., 1986, p. 52) have mentioned the use of rare-earth oxides as catalysts for methane oxidative coupling, with long contact times and preferably in the presence of a large excess of inert gas diluent.
The use of rare-earth oxides alone is disclosed in two patents (European patent 189 079 and World patent 8 607 351), and the use of rare-earth oxides doped with alkali metals is also disclosed in two patents (European patent 206 044 and World patent 8 607 351).
One disadvantage of the use of the above-mentioned catalysts results from their quick deactivation due to sintering phenomena, to the loss of volatile or liquid constituents such as alkali metals, or to coke deposition. Another disadvantage of the above-mentioned catalysts results from their low activity, requiring the use of high catalyst amounts, of long contact times and of a very high temperature, generally higher than 750.degree. C.